Which of the following are exothermic reactions? 1(i) CaO + H2O Ca(OH)2(ii) 2Pb(NO3)2Heat 2PbO + 4NO2 + O2(iii) CaCO3Heat CaO + CO2(iv) CH4 + 2O2 CO2 + 2H2O(A) (i) and (ii) (B) (ii) and (iii)(C) (i) and (iv) (D) (ii) and (iv)
Question
Which of the following are exothermic reactions? 1(i) CaO + H2O Ca(OH)2(ii) 2Pb(NO3)2Heat 2PbO + 4NO2 + O2(iii) CaCO3Heat CaO + CO2(iv) CH4 + 2O2 CO2 + 2H2O(A) (i) and (ii) (B) (ii) and (iii)(C) (i) and (iv) (D) (ii) and (iv)
Solution
Exothermic reactions are those that release heat. They have a negative enthalpy change.
(i) CaO + H2O Ca(OH)2: This is an exothermic reaction. When calcium oxide reacts with water, it forms calcium hydroxide and releases heat.
(ii) 2Pb(NO3)2Heat 2PbO + 4NO2 + O2: This is an endothermic reaction. The reaction requires heat to proceed, which is indicated by the "Heat" above the arrow.
(iii) CaCO3Heat CaO + CO2: This is also an endothermic reaction. The decomposition of calcium carbonate to calcium oxide and carbon dioxide requires heat.
(iv) CH4 + 2O2 CO2 + 2H2O: This is an exothermic reaction. The combustion of methane in oxygen releases heat and forms carbon dioxide and water.
So, the exothermic reactions are (i) and (iv). Therefore, the correct answer is (C) (i) and (iv).
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